1. Field of the Invention
The present invention relates to an objective lens configured to focus multiple laser beams having different wavelengths on corresponding optical discs and also to an optical pickup apparatus including the objective lens.
2. Description of the Related Art
There are multiple standards for optical discs used as information recording media. The laser beam, the recording density, and the thickness of a coating layer for coating an information recording layer, which are used by the standards, are different among the standards. To put it specifically, the optical discs of the compact disc (CD) standard, the digital versatile disc (DVD) standard and the blue-ray disc (BD) standard are currently used.
In order to reduce the number of components, a single objective lens preferably supports the optical discs of these three standards. However, the occurrence conditions of spherical aberration, comatic aberration, chromatic aberration and the like on the discs are different among the standards. Thus, it is not easy to develop an objective lens with these conditions taken into consideration.
To address this issue, there have been made approaches shown in Japanese Patent No. 4531649 and Japanese Patent Application Publication No. 2010-20899, for example.
Japanese Patent No. 4531649 discloses an optical pickup apparatus supporting the optical discs of the BD, DVD and HD DVD standards by using the refraction effect of an objective lens. To put it specifically, referring to paragraphs [0046] to [0048] in this publication, the lens is designed to have a shape causing only small RMS wavefront aberrations in the HD DVD standard and the DVD standard. In addition, as described in paragraph [0049], the RMS wavefront aberration in the CD standard is reduced through adjustment of the degree of divergence of incident light.
Japanese Patent Application Publication No. 2010-20899 discloses that the spherical aberration occurring due to a difference in thickness between coating layers of optical discs is offset by the chromatic aberration occurring due to a difference in wavelength between laser beams, which thus improves the aberration as the entire objective lens. To put it specifically, referring to paragraphs [0027] to [0032] in this publication, the value of wavefront aberration of the laser beam of the CD standard and the value of wavefront aberration of the laser beam of the DVD standard are each set to a predetermined value by forming the lens surface into a predetermined shape.